Extracting power from water has primarily come in the form of utilizing the potential energy of dammed water to drive a generator. A less common and greatly underutilized method of extracting power from water makes use of the kinetic energy contained in tidal and river flows. Harnessing power from the flow of water can provide an urgently needed, clean, renewable, and even inexhaustible energy source.
Various approaches have been taken to harness tidal energy. One approach has been to build large estuarine tidal barrages or tidal fences. At high tide, the water is either blocked behind a fence or shut into a sequence of locks and gates. Then, as the tide recedes, the trapped water is forced to escape through turbines to produce electric power. Tide barrages and tidal fences are large-scale, high-cost projects that must be custom designed for each particular site. They inhibit fish migration, induce silt build-up, cause localized flooding, and have an adverse environmental impact in general.
Another approach has been to use tidal turbines, which are analogous to wind turbines. Wind turbine manufacturers typically specify a cut-in speed, cut-out speed, and survival speed for their machines. The cut-in speed is the minimum fluid speed required for power production, the cut-out speed is the maximum fluid speed at which power can be produced, and the survival speed is the maximum fluid speed that the wind turbine can withstand without sustaining damage. Analogous to wind turbines, tidal turbines also have a cut-in speed, cut-out speed, and survival speed.
At sites with high average kinetic tidal power content (tidal power content), the maximum and minimum tidal current speed (tidal speed) may differ greatly. For example, at a certain site, the tidal speed may vary between 1 m/s and 8 m/s, with the average tidal speed around 4 m/s.
Conventional turbines must be designed to withstand the forces generated by the site's maximum tidal speed; in other words, a conventional turbine's survival speed must be larger than the maximum tidal speed at the site where it is deployed. Therefore it is not cost effective to build a conventional turbine at sites with large tidal speed variation. The conventional turbine and associated support structure must be sized for the maximum tidal speed, however power will only be produced near the average tidal speed and between the cut-in and cut-out speeds which are much slower and occur most frequently. Installation of a conventional tidal turbine at a site with large tidal speed variation requires an over-built and consequently expensive structure which can withstand the maximum tidal speed, and therefore this approach will not enable optimal return on investment.
In another approach, conventional tidal turbines have been placed at sites that have relatively narrow ranges of tidal speed variation. This approach requires extensive site characterization, and conventional turbines are typically custom-designed for such sites once flow properties are quantified. Even so, conventional turbines must still be built robust enough to withstand the drag associated with the peak tidal speed at such sites.
Both of these approaches result in an expensive project cycle which is likely to require site characterization, site-specific system development, and on-site construction; they preclude the economies of scale afforded by standardization and high-volume manufacturing. Because of the highly variable tidal speeds associated with high tidal power content sites, the kinetic tidal energy at such sites remains unharnessed. An inexpensive, unconventional tidal turbine which can regulate mechanical power output reliably between its cut-in and cut-out speeds, survive extremely high tidal speeds, eliminate the need for extensive site characterization, and be deployed at multiple high tidal power content sites is required.